The U.S. Army Corps of Engineers, St. Paul District, ended its plans for a 2006 Pool 5 drawdown last week.
Recent surveys of the main navigation channel in Pool 5 indicate there has been some in-filling of the channel at key locations. Hence, if the pool were in drawdown mode, it would make it difficult to maintain the authorized nine-foot navigation channel and pose an unacceptable risk to commercial navigation. After consulting with key internal and external technical experts, the drawdown team felt the potential negative impact on commercial navigation outweighed the benefits of continuing a drawdown.
The drawdown began on June 12, as scheduled, and was intended to restore and enhance habitat in the pool. The drawdown reached the target of 1.5 foot at Lock and Dam 5, and 0.5 foot at the Alma, Wis., gauge on June 26. Shortly thereafter, the Corps of Engineers began raising the pool elevation at the dam due to low and declining river flows.
There was a channel closure in Pool 5 at the Lower Zumbro River due to the grounding of a tow on July 8. A pilot channel was dredged that weekend and navigation resumed July 10.
Because the pool was very near normal pool elevations with minimal drawdown at the time of the grounding (less than 0.2 foot), the drawdown had no effect on this grounding. The channel closure was not a result of the low flow conditions. The pool was in normaal pool regulation mode by July 9. The drawdown lasted 27 days.
The extensive monitoring program for mussel impacts and vegetation response is continuing as planned. We expect to gain critical, valuable information on the effects of the 2005 and 2006 drawdowns in Pool 5.
The Water Level Management Task Force, a partner group involved in the drawdown program, will meet in the near future to discuss the 2006 drawdown and possibly plan for future drawdowns in Pool 5.